Self-regulating electrical heating cable

ABSTRACT

A series resistance heating cable comprises a heating element extending longitudinally along the cable. The element comprises a material having a positive temperature coefficient.

BACKGROUND

1. Field of the Invention

The present invention relates to an electrical heating cable, the poweroutput of which is self-regulating as the result of the incorporation ofa material with a positive temperature coefficient (PTC), as well asheating devices incorporating such cables.

2. Related Art

Parallel resistance semi-conductive, self-regulating heating cables arewell known. Such cables normally comprise two conductors (known asbuswires) extending longitudinally along the cable. Typically, theconductors are imbedded within a semi-conductive polymeric heatingelement, the element being extruded continuously along the length of theconductors. The cable thus has a parallel resistance form, with powerbeing applied via the two conductors to the heating element connected inparallel across the two conductors. The heating element usually has apositive temperature coefficient. Thus as the temperature of the elementincreases, the resistance of the material electrically connected betweenthe conductors increases, thereby reducing power output. Such heatingcables, in which the power output varies according to temperature, aresaid to be self-regulating or self-limiting.

FIG. 1 illustrates a typical parallel resistance, semi-conductive,self-regulating heating cable 2. The cable consists of a semi-conductivepolymeric matrix 8 extruded around the two parallel conductors 4, 6. Thematrix serves as the heating element. A polymeric insulator jacket 10 isthen extruded over the matrix 8. Typically, a conductive outer braid 12(e.g. a tinned copper braid) is added for additional mechanicalprotection and/or use as an earth wire. Such a braid is typicallycovered by a thermo plastic overjacket 14 for additional mechanical andcorrosive protection.

Such parallel resistance self-regulating heating cables possess a numberof advantages over non self-regulating heating cables, and are thusrelatively popular. For instance, self-regulating heating cables do notusually overheat or burn out due to their PTC characteristics. As thetemperature at any particular point in the cable increases, theresistance of the heating element at that point increases, reducing thepower output at that point, such that the heater is effectively switchedoff.

Further, due to this self-regulation of heating element temperature, itis often unnecessary to utilise “cold leads” with such heaters. Coldleads are often required in non-regulated heaters, as in a hightemperature environment, the heating element may reach relatively hightemperatures. Cold leads are connected to the ends of such non-regulatedheaters to enable the heating element to be connected to the electricalsupply without, for example, overheating the terminals or the supply.Cold leads typically take the form of relatively low resistance wiresarranged to produce no appreciable heat. However, the fixing of the coldleads often involves costly labour. Further, the connection between thecold lead and the heater has a relatively high failure rate, due to thetemperature gradient and thermal cycling experienced by the connection.

Consequently, as self-regulating heaters are typically arranged tooperate within a safe temperature range, cold leads are not required.

However, parallel resistance semi-conductive self-regulating heaters dopossess a number of undesirable characteristics.

The most common failure mode of parallel resistance self-regulatingheaters is loss of, or reduction in, electrical contact between thepower conductors and the extruded semi-conductive matrix forming theheating element. For example, differential expansion of the componentsand thermal cycling may lead to such failure or reduction in electricalcontact. Such a reduction leads to electrical arcing within the cable,and a consequent loss in thermal output. The operational life of theproduct is thus dependant upon the bond between the conductors and theheating element.

Often the heating cable will be at a relatively low temperature (andhence low resistance) when initially energised. The low resistance willthus draw a high start up current when the cable is energised from cold.Consequently, circuit breakers intended to provide a first level ofelectrical safety (over current protection) must be sized to allow muchhigher currents (often by a factor of 6) than the normal run oroperating current. This results in a lowering of circuit safety andover-sized switch gear and components.

SUMMARY

It is an object of the present invention to provide an electric heatingcable that substantially obviates or mitigates one or more of theproblems of the prior art, whether referred to herein or otherwise.

According to a first aspect, the present invention provides a seriesresistance heating cable comprising a heating element extendinglongitudinally along the cable, the element comprising a material havinga positive temperature coefficient.

By providing a self-regulating heating cable having a seriesarchitecture, the life expectancy of the cable is increased. Further,the start up current decreases compared with a similar parallelresistance self-regulating heating cable.

The cable may be a self-regulating cable.

The material may be a semi-conductor.

The material may comprise a polymer.

The material may comprise a high density polyethylene matrix includingcarbon.

The heating cable may further comprise at least one conductive terminallocated at an end of the cable, and in electrical contact with theheating element via a conductive paste.

The conductive paste may comprise silver.

According to a second aspect, the present invention provides a heatingdevice comprising a heating cable as described above.

The heating device may be a car seat heater.

According to a third aspect, the present invention provides a method ofmanufacturing a series resistance heating cable, the method comprisingthe step of providing a heating element extending longitudinally alongthe cable, the element comprising a material having a positivetemperature coefficient.

According to a fourth aspect, the present invention provides a method ofmanufacturing a heating device, the method comprising providing a seriesresistance heating cable having a heating element extendinglongitudinally along the cable, the element comprising a material havinga positive temperature coefficient.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

FIG. 1 is a partially cut away perspective view of a known parallelresistance self-regulating heating cable;

FIG. 2 is a partially cut away perspective view of a cable in accordancewith an embodiment of the present invention;

FIG. 3 is an end view of a terminal for connecting to the cableillustrated in FIG. 2;

FIGS. 4A and 4B illustrate the terminal of FIG. 3 being connected to thecable of FIG. 2; and

FIG. 5 is a schematic representation of a heating device in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION

The present inventor has realised that a series resistanceself-regulating heating cable combines the benefits of the parallelresistance self-regulating heating cables, but with less disadvantages.

FIG. 2 illustrates a series resistance self-regulating heating cable inaccordance with an embodiment of the present invention. The heatingcable 20 comprises a heating element 22 extending longitudinally alongthe cable. The heating element 22 has a positive temperaturecoefficient, such that resistance of the element increases withtemperature. Preferably, the element comprises a semi-conductivematerial shaped as a wire or string. One example of a suitable materialis semi conductive high density polyethylene (HDPE), such as carbonloaded polyethylene. Typically, the element will have a substantiallycircular cross section, of diameter 2 mm.

A primary insulation jacket or coating 24 surrounds the heating element22, and is used to electrically insulate the element 22 from thesurroundings. Typically, this primary insulation jacket 24 is formed ofa polymer such as polyolefin, of approximate thickness 0.8 mm.

A conductive outer braid 26 (e.g. copper braid typically of approximatethickness 0.5 mm) can optionally be added for additional mechanicalprotection and/or use as an earth wire. Such a braid may also be coveredby a thermo plastic outer jacket for additional mechanical protection,typically of approximate thickness 0.6 mm.

Although series resistance heating cables are known, such cablescomprise a metallic heating resistance wire having a substantiallyconstant electrical resistance. Such cables thus have a substantiallyconstant power output, irrespective of the temperature of the heater. Inhigh temperature environments, such series heaters continue to producethe designed heating load, which may result in over-heating or burn outof the heater unless externally controlled. This is a major disadvantageof known series resistance heaters.

However, by providing a heating element with a positive temperaturecoefficient, then when any portion of the heater is subjected to a hightemperature, power output from the heater is reduced to preventover-heating or burn out. Further, because the described embodiment isself-regulating, it may be arranged for connection directly to powersupply terminals without the need to fix separate cold leads. Thisobviates the attendant material and labour costs, and removes thepossibility of failure at a hot/cold joint. Preferably, the heatingelement is formed of polymeric and/or semi-conductive material. Suchmaterials are particularly suitable for self-regulating heater cables,as they have a relatively large PTC. In other words, the resistance ofthe material changes significantly for a predetermined temperaturerange. For instance, the resistance may change by 50% over a 100° C.temperature range. In polymeric materials, this change in resistance istypically due to the polymer expanding and at least partially breakingthe conductive path between the two conductors.

In addition to the aforesaid advantages of the embodiment, which aretypically shared by the parallel resistance self-regulating heatingcable, other advantages also arise due to the series architecture.

Compared with a similar parallel-resistance self-regulating heatingcable, a series resistance self-regulating heating cable experiences alower inrush current on cold start up. This is because the inrushcurrent is inversely proportional to the distance that separates thelive and neutral terminals. In a parallel cable the two conductors areclose together, typically 8 mm apart. The applied mains voltage caneasily ‘jump’ across the two buswires via the carbon loadedsemi-conductor. Conversely, in the series architecture, the twoterminals are some distance apart, typically metres as opposed tomillimeters, and hence inrush is inhibited. For example, a typicalparallel-resistance self-regulating heating cable rated at 30 watts permeter might have a cold start resistance of approximately 300Ω, risingto a stable resistance of around 2 kΩ after a predetermined time period.In other words, the resistance of the cable changes by at least an orderof magnitude. In contrast, a similarly rated series resistance cablemight have a cold start resistance of 1-1.5Ω, rising to a stableresistance of 2 kΩ. It-will this be appreciated that the resistancechange of the series cable is lower than the resistance change of thesimilar parallel cable, with the series cable thus having a lower inrushcurrent on cold start up. Consequently, over-current protection devicesmay therefore be sized closer to the operating current, therebyimproving circuit safety, and decreasing the amount by which switch gearand components have to be over-sized. Additionally, series resistanceself-regulating heating cables are less susceptible to failure thanparallel resistance self-regulating heating cable. This is because in aseries self-regulating heating cable, good electrical contact betweenthe power conductors and the element need only be made at the two endsof the series cable, as opposed to a continuously good contact along thewhole length of the parallel cable. Further, as the contacts with theconductors are made at the end of the cable, should repair orreplacement of the contact prove necessary, this is readilyaccomplished.

FIG. 3 shows an end view of a terminal 30 suitable for making anelectrically conductive connection with an end of the heating cable.Preferably, a similar connection is made at each end of the cable. FIG.4A illustrates a cross sectional view of the terminal being applied tothe heating element 22 located at one end of the cable 20, whilst FIG.4B illustrates the terminal in situ. The terminal is connected to aconductive lead (not shown), which is in turn connected to a powersupply suitable for supplying power to operate the heater.

The terminal 30 comprises a body 32 defining an aperture. Legs 34 extendaway from the body 32. Located at an end of each leg distant from thebody 32 is a jaw 36. In use, the jaw 36 is arranged to dig into and gripa surface e.g. the jaw 36 is arranged to be imbedded within the surfaceof the heating element 22.

As can be seen in FIGS. 4A and 4B, the terminal 30 is located with thebody 32 adjacent an end of the longitudinally extending heating element22. The legs 34 extend along the sides of the heating element 22. Aconductive paste (e.g. a silver paste) is injected through the aperture(in the direction shown by arrow 38) in the body 32, so as to fill thevoid between the end of the cable and the adjacent surface of theterminal body 32. Subsequently, the paste is set, ensuring a goodelectrical contact between the terminal and the heating element. Shouldelectrical contact be lost, a new conductive paste coating may bereadily applied.

Additionally, pressure is applied to the ends of the legs 34 distantfrom the body 32, so as to embed the jaws 36 within the element 22.

Such a series resistance self-regulating heating cable is suitable foruse on a variety of heating devices and applications. It is particularlysuitable for use in devices of known, predetermined length. This enableseasier sizing of the heating device.

It has been appreciated by the present inventor that series resistanceself-regulating heating cables comprising PTC materials are particularlysuitable for use in heating devices or arrangements in which it isdesirable to selectively heat a portion of the device in contact with anexternal body e.g. car seat heaters or motor cycle handle bar gripheaters. One example of such a material is carbon loaded polyethylene.

FIG. 5 illustrates a plan view of a car seat heater arrangement, showingthe layout of the series resistance self-regulating heating cable 20within the car seat heater 40.

The overall width A of the heater 40 is approximately 600 mm with alength B of approximately 900 mm. Apart from the ends of the cableprovided with a terminals 30 for connection to a power supply, the cable20 is distributed so as to maintain a distance of at least C from theperiphery of the heater. Typically, C is 100 mm. The cable is arrangedwithin the car seat heater so as to be substantially evenly distributedwithin the car seat, with typical cable spacing being D, a distance ofapproximately 100 mm.

Such an arrangement tends to provide a total cable length ofapproximately 3000 mm. For a 3 W/m rated cable, this typically resultsin a circuit resistance of approximately 16 ohms, whilst for a 7 W/mcable this would result is a circuit resistance of approximately 6.9ohms.

In normal operation, the cable will emit heat, so as to warm the carseat. If a user contacts the surface overlying a portion of the cable,then this will result in an increase in temperature of that portion dueto the rate of heat loss being decreased by the relatively warm body ofthe user. It will consequently be realised that, due to the elementcomprising a PTC material, the areas of the seat contacted by a user(eg. sat upon) will experience an increase in resistivity. This increasein resistivity will lower the overall heat output from the cable, as thetotal resistance of the cable will increase. However, for those areas inwhich the resistance increases, due to the serial nature of the cable,the heat emitted from those areas will be higher than the heat emittedfrom other, lower resistance areas where the users body mass is notpresent. The use of a series resistance cable with a PTC will thusprovide a heater in which the majority of the heat is emitted from thearea contacted by a user, whilst the PTC ensures this area is onlymaintained at a reasonable temperature that does not burn the user.

1. A series resistance self-regulating heating cable comprising: aheating element extending longitudinally along the cable, wherein theheating element comprises a semi-conductor having a positive temperaturecoefficient; and at least one conductive terminal coupled to an outsidesurface of the cable, electronically connected to the heating element,and located at an end of the cable.
 2. The heating cable as claimed inclaim 1, wherein said semi-conductor comprises a polymer.
 3. The heatingcable as claimed in claim 1, wherein said semi-conductor comprises ahigh density polyethylene matrix including carbon.
 4. The heating cableas claimed in claim 1, the at least one conductive terminal being inelectrical contact with the heating element via a conductive paste. 5.The heating cable as claimed in claim 4, wherein said conductive pastecomprises silver.
 6. A heating device, comprising: a series resistanceself-regulating heating cable including a heating element extendinglongitudinally along the cable, wherein the heating element includes: asemi-conductor having a positive temperature coefficient; and at leastone conductive terminal coupled to an outside surface of the cable,electronically connected to the heating element, and located at an endof the cable.
 7. The heating device as claimed in claim 6, wherein saidheating device is a car seat heater.
 8. A method of manufacturing aseries resistance self-regulating heating cable, comprising: extending aheating element longitudinally along the cable, wherein the heatingelement includes: a semi-conductor having a positive temperaturecoefficient; and at least one conductive terminal coupled to an outsidesurface of the cable, electronically connected to the heating element,and located at an end of the cable.
 9. A method of manufacturing aheating device, comprising: producing a series resistanceself-regulating heating cable having a heating element extendinglongitudinally along the cable, wherein the heating element includes: asemi-conductor having a positive temperature coefficient; and at leastone conductive terminal coupled to an outside surface of the cable,electronically connected to the heating element, and located at an endof the cable.